A Cartesian loop transmitter is used as a high output linear amplifier/transmitter. FIG. 3 shows an example of configuration of a conventional type of Cartesian loop transmitter comprising a DSP 301 (digital signal processor) which executes processing for an input signal such band limitation, band division and frequency shift, all required for tone-in-band generation. An output of the DSP signal is divided to two signals I and Q crossing each other at right angles, and applied to D/A convertors 302, 303 which convert the output signals I, Q (digital signals) from the DSP 301 to analog signals respectively. An LPF (low pass filter) 304 receives the signal from the D/A convertor 302 together with a signal I.sub.DC from an operational amplifier 313, described later, and subjects the signals to band limitation and then outputs the signal. An LPF 305 receives a signal from the D/A convertor 303 together with a signal Q.sub.DC from an operational amplifier 314, described later, subjects the signals to band limitation and then outputs the signals. A local oscillator 306 outputs a local signal (carrier wave) L.sub.O, and a quadrature modulator 307 receives signals from the LPFs 304, 305 and local oscillator 306 and outputs modulated wave after quadrature modulation. A power amplifier 308 amplifies the modulated wave outputted from the quadrature modulator 307. An antenna 309 transmits the modulated wave amplified in the power amplifier 308. An ATT (attenuator) 310 detects only a progressive wave from the modulated wave amplified in the power amplifier 308 and executes attenuation for level adjustment. A local oscillator 311 outputs a local signal (carrier wave) L.sub.O, and a quadrature modulator 312 into which signals from the ATT 310 and the local oscillator 311 are inputted, subjects the signals to quadrature demodulation. An operational amplifier 313 amplifies signals from the quadrature demodulator 312 and outputs a signal I.sub.DC, and an operational amplifier 314 amplifies the signals from the quadrature demodulator 312 and outputs a signal Q.sub.DC. It should be noted that, in the figure, the reference numeral 315 indicates an adder while the reference numeral 316 indicates a multiplier.
As clearly indicated by the configuration described above, the Cartesian loop transmitter has a negative feedback circuit to return output from the quadrature modulator 307 via the quadrature demodulator 312 to an input terminal of the quadrature modulator 307 again, so that distortion due to non-linear amplification is reduced, and thus the function as an amplifier/transmitter is executed.
However, with the conventional type of Cartesian loop transmitter as described above, carrier (carrier wave) leak generated by the quadrature modulator and the quadrature demodulator is transmitted from the antenna as an unnecessary interference signal together with a transmission signal, so that communication quality is disadvantageously deteriorated.
Specifically, in the quadrature modulator, carrier leak is generated due to incomplete isolation to a local signal inputted from a local oscillator. Also due to incomplete isolation in the quadrature demodulator for feedback, a DC element in the I, Q channels is amplified, a carrier signal is generated as an input signal from the quadrature demodulator, and this carrier signal is generated as carrier leak at an output terminal of the quadrature demodulator.